U.S. patent application number 12/919687 was filed with the patent office on 2011-05-12 for flash light annealing for thin films.
This patent application is currently assigned to The Trustees of Columbia University in the City of. Invention is credited to Ui-Jin Chung, James S. Im, Paul C. Van Der Wilt.
Application Number | 20110108108 12/919687 |
Document ID | / |
Family ID | 41056568 |
Filed Date | 2011-05-12 |
United States Patent
Application |
20110108108 |
Kind Code |
A1 |
Im; James S. ; et
al. |
May 12, 2011 |
FLASH LIGHT ANNEALING FOR THIN FILMS
Abstract
A method of making a crystalline film includes providing a film
comprising seed grains of a selected crystallographic surface
orientation on a substrate; irradiating the film using a pulsed
light source to provide pulsed melting of the film under conditions
that provide a mixed liquid/solid phase and allowing the mixed
solid/liquid phase to solidify under conditions that provide a
textured polycrystalline layer having the selected surface
orientation. One or more irradiation treatments may be used. The
film is suitable for use in solar cells.
Inventors: |
Im; James S.; (New York,
NY) ; Van Der Wilt; Paul C.; (New York, NY) ;
Chung; Ui-Jin; (Rego Park, NY) |
Assignee: |
The Trustees of Columbia University
in the City of
New York
NY
|
Family ID: |
41056568 |
Appl. No.: |
12/919687 |
Filed: |
February 27, 2009 |
PCT Filed: |
February 27, 2009 |
PCT NO: |
PCT/US09/35537 |
371 Date: |
January 25, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61032781 |
Feb 29, 2008 |
|
|
|
61111518 |
Nov 5, 2008 |
|
|
|
Current U.S.
Class: |
136/258 ; 257/75;
257/E21.328; 257/E29.003; 257/E31.043; 438/795; 438/97 |
Current CPC
Class: |
H01L 21/02667 20130101;
H01L 31/0236 20130101; H01L 21/02532 20130101; Y02P 70/50 20151101;
H01L 31/028 20130101; H01L 31/072 20130101; Y02E 10/547 20130101;
H01L 31/03762 20130101; H01L 31/1872 20130101; H01L 31/03921
20130101; Y02P 70/521 20151101; H01L 21/02686 20130101; Y02E 10/548
20130101; H01L 31/1804 20130101; H01L 21/02609 20130101; H01L
21/2686 20130101; H01L 21/02425 20130101; H01L 21/67115
20130101 |
Class at
Publication: |
136/258 ;
438/795; 438/97; 257/75; 257/E21.328; 257/E31.043; 257/E29.003 |
International
Class: |
H01L 31/0368 20060101
H01L031/0368; H01L 21/26 20060101 H01L021/26; H01L 31/18 20060101
H01L031/18; H01L 29/04 20060101 H01L029/04 |
Claims
1. A method of making a crystalline film, comprising: providing a
film comprising seed grains with a substantially uniform
crystallographic surface orientation on a substrate; irradiating
the film using a pulsed light source to provide pulsed melting of
the film under conditions to provide a plurality of solid sections
and liquid sections extending throughout the thickness of the film,
creating a mixed liquid/solid phase comprising one or more of the
seed grains; and allowing the mixed solid/liquid phase to solidify
from the seed grains to provide a textured polycrystalline layer
having the crystallographic surface orientation of the seed
grains.
2. The method of claim 1, wherein providing a film comprises:
providing an amorphous film; and subjecting the amorphous film to a
radiation-induced transformation to polycrystalline silicon prior
to the creation of a mixed liquid/solid phase to provide a film
comprising seed grains of the substantially uniform
crystallographic surface orientation.
3. The method of claim 1, wherein the mixed solid/liquid phase has
a periodicity approaching a critical solid-liquid coexistence
length (X.sub.is).
4. The method of claim 1, wherein the selected surface orientation
is a {100} plane.
5. The method of claim 1, wherein the resultant textured
polycrystalline layer comprises about 90% of the surface area of
the film having a {100} surface orientation within at least one of
about 15.degree. of the {100} pole, about 10.degree. of the {100}
pole, and about 5.degree. of the {100} pole.
6. The method of claim 1, wherein the conditions of irradiation are
selected to provide an intensity of incident light to provide a
periodicity of the liquid-solid phase that approaches
.lamda..sub.ls.
7. The method of claim 1, wherein the pulsed light source is a
divergent light source.
8. The method of claim 7, wherein the pulsed divergent light source
comprises at least one of a flash lamp and a laser diode.
9. The method of claim 1, wherein the film comprises silicon.
10. The method of claim 1, wherein the liquid content of the mixed
solid/liquid phase is in the range of at least one of about 50 vol
% to less than 100 vol % and about 80 vol % to about 99 vol.
11. The method of claim 1, wherein the intensity of the divergent
light source pulse is selected to provide a mixed solid/liquid
phase.
12. The method of claim 1, wherein the film thickness is in the
range of at least one of about 50 nm to about 1 .mu.m and about 150
nm to about 500 nm.
13. The method of claim 1, wherein the film is exposed to at least
one of a single flash lamp pulse and multiple light pulses.
14. The method of claim of claim 13, wherein a second and
subsequent pulse has a higher energy density than the first light
pulse.
15. The method of claim 13, wherein second and subsequent pulses
are more than 20% higher energy density than the first light
pulse.
16. The method of claim 13, wherein the layer is exposed to at
least one of one of 2-10 light pulses and 2-4 light pulses.
17. The method of claim 1, wherein the light source pulse provides
a liquid/solid mix having at least about 50 vol % liquid.
18. The method of claim 1, wherein the energy intensity of the
incident light is about 2 J/cm.sup.2 to about 150 J/cm.sup.2.
19. The method of claim 1, wherein the mixed liquid/solid phase is
achieved by selection of energy density, pulse shape, dwell time,
and wavelength of the light incident to the film.
20. The method of claim 1, further comprising preheating the
substrate prior to flash lamp irradiation.
21. The method of claim 21, wherein the light source comprises at
least a wavelength in the range of 400-900 nm.
22. The method of claim 21, wherein the light source comprises
light of a wavelength selected for absorption by one or more of an
underlying heat absorption layer and the film.
23. The method of claim 1, wherein the light source comprises white
light.
24. The method of claim 1, further comprising providing a metal
underlayer for the film, wherein the heat of the light source is at
least partially absorbed by the metal layer.
25. The method of claim 24, wherein a barrier layer is disposed
between the film and the metal layer to reduce interaction of the
film with the metal layer.
26. The method of claim 24, wherein the metal layer is patterned to
provide heat absorption in selected areas.
27. The method of claim 1, further comprising: irradiating the
mixed liquid/solid phase with the pulsed light source.
28. The method of claim 1, wherein the thin film is divided into
one or more isolated sections.
29. The method of claim 28, wherein the substrate comprises one or
more trenches proximate to one or more of the isolated
sections.
30. A method of making a crystalline film, comprising: providing a
film comprising seed grains of a substantially uniform
crystallographic surface orientation on a substrate; irradiating
the film using a pulsed light source to provide pulsed melting of
the film under conditions to provide a plurality of liquid sections
and solid sections extending throughout the thickness of the film,
creating a mixed liquid/solid phase having a periodicity of less
than the solid-liquid coexistence length (.lamda..sub.ls) and
comprising one or more of the seed grains; allowing the mixed
solid/liquid phase to solidify from the seed grains under
conditions that provide a textured polycrystalline layer having the
selected surface orientation; and irradiating the film using a
second pulsed light source to provide pulsed melting of the film
under conditions that provide a plurality of solid sections and
liquid sections extending throughout the thickness of the film,
creating a mixed liquid/solid phase having a periodicity of greater
than formed in the first pulse; and allowing the mixed solid/liquid
phase to solidify under conditions that provide a textured
polycrystalline layer having the selected surface orientation,
wherein at least one of the surface texture, grain size, and
defectivity is improved in the second pulsed irradiation.
31. The method of claim 30, wherein at least one grain remains in
the film after the first pulsed irradiation that is different from
the selected surface orientation, and wherein the number of said
different grains is reduced in the film after the second
irradiation pulse.
32. The method of claim 30, wherein each of the first pulsed light
source and the second pulsed light source comprise a divergent
light source.
33. A method of forming a solar cell, comprising: (a) providing a
textured seed layer by: providing a silicon film comprising seed
grains of a {100} surface orientation on a substrate; irradiating
the film using a pulsed divergent light source to provide pulsed
melting of the film under conditions that provide a plurality of
solid sections and liquid sections extending throughout the
thickness of the film, creating a mixed liquid/solid phase having a
periodicity of a critical solid-liquid coexistence length
(.lamda..sub.ls); and allowing the mixed solid/liquid phase to
solidify under conditions that provide a textured polycrystalline
layer having the {100} surface orientation; and (b) epitaxially
growing a polycrystalline silicon layer on the textured seed layer
to form a textured film.
34. A textured polycrystalline film disposed on a glass substrate,
the film having at least 90% of the surface area of the film on a
glass substrate oriented to within about 15.degree. of the {100}
pole.
35. A crystalline film produced by the method of claim 1.
36. A crystalline film produced by the method of claim 30.
37. A solar cell produced by the method of claim 33.
Description
RELATED APPLICATIONS
[0001] This application is related to co-pending, commonly owned
application Ser. No. 61/111,518, filed Nov. 5, 2008, and Ser. No.
61/032,781, filed Feb. 29, 2008, and incorporated in its entirety
by reference.
FIELD
[0002] The disclosed subject matter generally relates to
crystallization of thin films and particularly relates to using a
pulsed flood light source in such crystallization.
BACKGROUND
[0003] Some solar cells use crystallized silicon films to conduct
carriers. Solar cells use minor carriers, and in order to have a
reasonable efficiency, they require films with a low defect
density. The defects in a crystallized silicon film include grains
boundaries, i.e., the boundaries between the crystallographic
grains, as well as intragrain defects, i.e., the defects within the
crystallographic grains, such as twin boundaries and stacking
faults. To improve the efficiency of the solar cells, it is
desirable to reduce the density of grain boundaries, that is, to
increase the size of these grains, as well as reducing the density
of intragrain defects.
[0004] Presently the most common method of making solar cells
employs single crystal silicon (c-Si) substrates. These wafers
provide a high quality substrate, but are expensive due to limited
silicon feedstock availability. Polycrystalline silicon (poly-Si)
substrates, e.g., from ingots, can be used but have only slightly
lower cost. The current trend is to reduce the thickness of the
c-Si and poly-Si wafer-based solar cells (for example below 200
.mu.m); however, challenges arise regarding the mechanical
properties of such wafers, for example in handling during
processing.
[0005] Thin-film amorphous and/or nanocrystalline silicon solar
cells use significantly less silicon, which has a potential cost
advantage. Furthermore, they can be deposited on large-area
substrates such as glass, metal foils, or even plastics. However,
amorphous silicon still suffers from poor stability and lower
efficiency than crystalline silicon. Thin film polycrystalline
solar cells could potentially form an attractive compromise by
offering low cost through limited use of silicon, while offering
high stability and efficiency through the use of crystalline
silicon.
[0006] To form thin-film polycrystalline films, an amorphous
silicon (a-Si) layer can be treated to induce crystallization, for
example, using thermal annealing techniques. However, such solid
phase crystallization methods are known to result in films with a
high intragrain defect density, and furthermore, they require long
time periods and high temperatures, making them less suitable for
thermally sensitive substrates such as glass.
[0007] Poly-Si films have been prepared using a seed layer
approach. This approach starts from a low cost large substrate and
creates a thin seed crystalline layer on top of the substrate.
Conventional methods of obtaining a crystalline seed layer include
aluminum-induced crystallization. The method results in large grain
growth, but introduces many intragrain defects, so much so that
above a certain grain size (for example a few .mu.m) the properties
of the film are dominated by the intragrain defects. Thus, the
layer acts like a small grained material. In addition, the texture
that is achieved in the process is relatively poor, for example
only 75% of the surface area is within 20 degrees of the {100}
pole. In a subsequent step, a thick crystalline layer is grown from
the seed layer using epitaxial growth methods, such as plasma
enhanced chemical vapor deposition. Low temperature chemical vapor
deposition methods, such as hot wire chemical vapor deposition
(CVD), are attractive as they offer the potential of glass
compatibility; however, at low temperatures, these methods require
high quality {100} oriented surfaces for qualitative epitaxial
growth.
[0008] Zone-melting recrystallization (ZMR) of Si films can result
in the formation of large grained polycrystalline Si films having a
preferential {100} surface orientation of the crystals. The films
qualify as seed layers because they have a low defect density, that
is, large grain sizes, and a low number of intragrain defects.
Moreover, silicon films having a (100) surface texture can be
prepared. Such a texture is preferred for most epitaxial growth
processes performed at low temperatures. However, stable growth of
these long (100) textured grains is typically only observed at very
low scan rates that are not compatible with preferred low-cost
substrates such as glass.
[0009] Flash lamp annealing (FLA) has been used to crystallize an
amorphous silicon film. These lamps have a low cost and a high
power. In FLA, the flash discharge lamps produce a short-time pulse
of intense light that can be used to melt and recrystallize the
silicon layer. The FLA techniques used up to now have resulted in
crystallized silicon films with high defect densities. As a result,
these films are not optimal for use in solar cells. Thus, practical
techniques are still lacking for use of FLA methods to grow high
quality crystalline films.
SUMMARY
[0010] This application describes methods and systems for utilizing
flash lamp annealing (FLA) and other low cost divergent light
sources to crystallize films with large grains and low intragrain
defect density.
[0011] In one embodiment, a method of making a crystalline film
includes providing a film comprising seed grains with a
substantially uniform crystallographic surface orientation on a
substrate, irradiating the film using a pulsed light source to
provide pulsed melting of the film under conditions to provide a
plurality of solid sections and liquid sections extending
throughout the thickness of the film, creating a mixed liquid/solid
phase comprising one or more of the seed grains, and allowing the
mixed solid/liquid phase to solidify from the seed grains to
provide a textured polycrystalline layer having the
crystallographic surface orientation of the seed grains. The method
also can include providing a film, which includes providing an
amorphous film and subjecting the amorphous film to a
radiation-induced transformation to polycrystalline silicon prior
to the creation of a mixed liquid/solid phase to provide a film
comprising seed grains of the substantially uniform
crystallographic surface orientation.
[0012] In one or more embodiments, the periodicity of the mixed
liquid-solid phase has a periodicity approaching a critical
solid-liquid coexistence length (.lamda..sub.ls).
[0013] In one or more embodiments, the selected surface orientation
is a {100} plane.
[0014] In one or more embodiments, the resultant textured
polycrystalline layer comprises about 90% of the surface area of
the film having a {100} surface orientation within about 15.degree.
of the {100} pole, or the resultant textured polycrystalline layer
comprises about 90% of the surface area of the film having a {100}
surface orientation within about 10.degree. of the {100} pole, or
the resultant textured polycrystalline layer comprises about 90% of
the surface area of the film having a {100} surface orientation
within about 5.degree. of the {100} pole.
[0015] In one or more embodiments, the conditions of irradiation
are selected to provide an intensity of incident light to provide a
periodicity of the liquid-solid phase that approaches
.lamda..sub.ls.
[0016] In one or more embodiments, the pulsed divergent light
source comprises a flash lamp or a laser diode.
[0017] In one or more embodiments, the film comprises silicon.
[0018] In one or more embodiments, the liquid content of the mixed
solid/liquid phase is in the range of about 50 vol % to about 99
vol %, or about 80 vol % to about 99 vol %.
[0019] In one or more embodiments, the irradiating conditions are
selected to have a liquid content of the mixed solid/liquid phase
of greater than 80 vol % when the distance between seeds exceeds
.lamda..sub.ls, or the intensity of the divergent light source
pulse is selected to provide a mixed solid/liquid phase.
[0020] In one or more embodiments, the film thickness is in the
range of about 50 nm to about 1 .mu.m, or in the range of about 150
nm to about 500 nm.
[0021] In one or more embodiments, the method further includes
epitaxially growing a thick silicon layer on the textured
layer.
[0022] In one or more embodiments, the layer is exposed to a single
flash lamp pulse, and the light source pulse provides a
liquid/solid mix having at least about 90 vol % liquid.
[0023] In one or more embodiments, the layer is exposed to multiple
light pulses, such as in 2-10 light pulses or 2-4 light pulses.
[0024] In one or more embodiments, the light source pulse provides
a liquid/solid mix having at least about 50 vol % liquid.
[0025] In one or more embodiments, the energy intensity of the
incident light is about 2-150 J/cm.sup.2.
[0026] In one or more embodiments, the mixed liquid/solid phase is
achieved by selection of energy density, pulse shape, dwell time,
and wavelength of the light incident to the film.
[0027] In one or more embodiments, further comprises preheating the
substrate prior to flash lamp irradiation.
[0028] In one or more embodiments, the light source is of a
wavelength in the range of 400-900 nm, or the light source
comprises white light, or the light source comprises light of a
wavelength selected for absorption by the film, or the light source
comprises light of a wavelength selected for absorption by one or
more of an underlying heat absorption layer.
[0029] In one or more embodiments, further comprises providing a
metal underlayer for the film, wherein the heat of the light source
is at least partially absorbed by the metal layer.
[0030] In one or more embodiments, a barrier layer is disposed
between the film and the metal layer to reduce interaction of the
film with the metal layer.
[0031] In one or more embodiments, the metal layer is patterned to
provide heat absorption in selected areas.
[0032] In one or more embodiments, the film is pretreated to
provide seed grains of a selected orientation, and the seed grains
provided by a method selected from the group consisting of solid
phase anneal, pulsed laser crystallization and melt-mediated
explosive growth.
[0033] In one or more embodiments, the pulsed laser source is a
divergent light source.
[0034] In one or more embodiments mixed liquid/solid phase is
irradiated with the pulsed light source.
[0035] In one or more embodiments, the film is divided into one or
more isolated sections and can include one or more trenches
proximate to one or more of the isolated sections.
[0036] In one or more embodiments, a method of making a crystalline
film includes providing a film comprising seed grains of a
substantially uniform crystallographic surface orientation on a
substrate, irradiating the film using a pulsed light source to
provide pulsed melting of the film under conditions to provide a
plurality of liquid sections and solid sections extending
throughout the thickness of the film, creating a mixed liquid/solid
phase having a periodicity of less than the solid-liquid
coexistence length (.lamda..sub.ls) and comprising one or more of
the seed grains, allowing the mixed solid/liquid phase to solidify
from the seed grains under conditions that provide a textured
polycrystalline layer having the selected surface orientation and
irradiating the film using a second pulsed light source to provide
pulsed melting of the film under conditions that provide a
plurality of solid sections and liquid sections extending
throughout the thickness of the film, creating a mixed liquid/solid
phase having a periodicity of greater than formed in the first
pulse, and allowing the mixed solid/liquid phase to solidify under
conditions that provide a textured polycrystalline layer having the
selected surface orientation, wherein at least one of the surface
texture, grain size, and defectivity is improved in the second
pulsed irradiation.
[0037] In one or more embodiments, at least one grain remains in
the film after the first pulsed irradiation that is different from
the selected surface orientation, and wherein the number of said
different grains is reduced in the film after the second
irradiation pulse.
[0038] In one or more embodiments, the first and second pulsed
light sources are divergent light sources.
[0039] In another aspect of the invention, a method of forming a
solar cell is provided including (a) providing a textured seed
layer by providing a silicon film comprising seed grains of a {100}
surface orientation on a substrate; irradiating the film using a
pulsed divergent light source to provide pulsed melting of the film
under conditions that provide a pluarality of solid sections and
liquid sections extending throughout the thickness of the film ,
creating mixed liquid/solid phase having a critical solid-liquid
coexistence length (.lamda..sub.ls); and allowing the mixed
solid/liquid phase to solidify under conditions that provide a
textured polycrystalline layer having the selected surface
orientation; and (b) epitaxially growing a polycrystalline silicon
layer on the textured seed layer to form a textured film.
[0040] In another aspect of the invention, a textured
polycrystalline film is provided having at least 90% of the surface
area of the film oriented to within about 15.degree. of the {100}
pole.
[0041] The disclosed techniques, for example, can control the
heating cycle experienced by any location in the film. The
described methods and system can be used for creating seed layers
in an epitaxial growth process for making solar cells. These
methods and systems can enable the use of FLA and other low cost
divergent light sources, such as diode laser, for large scale
production of crystalline films for solar cells. The process may
further be used to create (100) textured films for use in
3D-ICs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] The disclosed subject matter is described with reference to
the following drawings which are presented for the purpose of
illustration only and are not intended to be limiting of what is
disclosed herein.
[0043] FIG. 1 is a schematic illustration of a flash lamp apparatus
that may be used, according to some embodiments of the disclosed
subject matter.
[0044] FIG. 2 is a cross-sectional illustration of a (A) melt
profile and corresponding temperature profile of a film having
homogeneous crystal morphology and (B) the resultant solidified
film, according to some embodiments of the disclosed subject
matter.
[0045] FIG. 2C is a graphical representation of a critical
solid-liquid coexistence length (.lamda..sub.ls) of a mixed
solid/liquid phase film, according to some embodiments of the
disclosed subject matter.
[0046] FIG. 3 is a cross-sectional illustration of (A) a film
having heterogeneous crystal morphology; and (B) a melt profile and
corresponding temperature profile of the heterogeneous film,
according to some embodiments of the disclosed subject matter.
[0047] FIG. 4 is a cross-sectional illustration of (A) a film
having a heterogeneous crystal morphology, (B) a melt profile and
corresponding temperature profile in which the periodicity
commensurate with .lamda..sub.ls is less than the spacing between
(100) grains so that some (hkl) grains survive; and (C) the
resultant solidified film, according to some embodiments of the
disclosed subject matter.
[0048] FIG. 5 is a plot of grain size vs. number of exposures,
illustrating the effect of multiple exposures on grain size,
according to some embodiments of the disclosed subject matter.
[0049] FIG. 6 is a plot of % (100) texture vs. number of exposures,
illustrating the effect of multiple exposures on texture size,
according to some embodiments of the disclosed subject matter.
[0050] FIGS. 7A and 7B are photomicrographs of an Si thin film that
has been crystallized using partial melt processing and continuous
wave complete melting, respectively, according to some embodiments
of the disclosed subject matter.
[0051] FIGS. 8A and 8B are schematics of a thin film
crystallization system implementing heat flow isolation, according
to some embodiments of the disclosed subject matter.
DETAILED DESCRIPTION
[0052] This application provides methods and systems to produce
high efficiency and low cost silicon thin films that are suitable
for use in solar cells. The application uses flash lamp technology
or other low cost pulsed flood light source, such as a diode laser,
to provide pulsed melting of a silicon film under conditions that
provide a mixed liquid/solid phase. The solid phase provides
seeding sites for the crystalline growth of silicon from the liquid
phase. Under appropriate conditions, a highly textured poly-Si
layer is obtained. In one or more embodiments, a poly-Si layer with
strong (100) texture is provided. The present application also uses
flash lamp annealing for creating seed layers in an epitaxial
growth process for making solar cells. It will be apparent from the
description that follows that the method is not limited to silicon
thin film crystallization and may be practiced for any thin film
that shows an increase in reflectivity upon melting. For the
purposes of discussion that follows, unless specifically noted, the
methods may be used for any such material. It also will be apparent
from the description that follows that other pulsed light sources
may be used, so long as they also provide a pulsed divergent light
source or a pulsed flood light and the desired control of the mixed
phase partial melting process. Unless explicitly stated, flash lamp
annealing or "FLA" is also meant to include diode lasers and other
divergent pulsed light sources used as a "flash lamp." Glass
compatibility may be very challenging with FLA, thus other
substrates are also considered for use in this process.
[0053] Partial melting zone melt recrystallization can be used to
provide crystalline films having (100) texture under favorable
conditions. In a conventional ZMR process, growth of the long (100)
textured grains starts on grains formed in the "transition region"
between the unmelted and the completely melted areas of the film.
This is the regime of partial melting in which regions that are
either solid or liquid throughout the thickness of the film
co-exist, and that only exists in radiatively heated Si films as a
result of a significant increase in reflectivity of Si upon melting
(a semiconductor-metal transition). In this partial melting regime,
{100} surface-oriented grains have been observed to dominate, a
phenomenon that is sometimes linked to a crystallographic
anisotropy in the Si0.sub.2--Si interfacial energy. As a result of
the negative feedback that results from the reduction in heat
coupling to the film from increased melting, the partial melting
regime is self stabilized and can be induced throughout the film by
radiation at beam intensities below what is required for complete
melting. This has been demonstrated in a partial melting ZMR
process using continuous wave laser scanning See, e.g., van der
Wilt, et al., "Mixed-Phase Zone-Melting Recrystallization of Thin
Si Films Via CW-Laser Scanning," Materials Science and Engineering,
Columbia University, March 2008, which is incorporated by
reference.
[0054] One limitation of the laser based ZMR processes is that the
light from lasers suffers from coherence, which makes it
challenging to create well-homogenized beams. Variation in the
power will lead to variation in the solid to liquid ratio in the
mixed phase and in a variation in the effectiveness of the process.
The non-uniformity in a line-beam created using a diffractive
optical element (DOE) can be as large as +/-15%. The melted zone is
often very narrow so that heat diffuses sideways through the film,
which then requires higher light intensity to compensate for heat
loss. However, this also gives rise to smaller grains. Another
limitation of the technique is the cost associated with the laser
technology. For most practical applications, a single laser head is
not powerful enough (up to e.g., 18W) and multiple heads need to be
integrated to create a sufficiently large and sufficiently powerful
beam. This will further add to system complexity and cost. Finally,
most lasers are also known to be inefficient sources of light in
which much power is used to create an often monochromatic source of
light.
[0055] Further, irradiation using a line-beam shaped pulse laser
source and a pulsed flood light source (i.e., using FLA) create
different surface morphologies in the thin film. Normally upon
lateral growth (e.g., with SLS), the lateral growth fronts collide
and a protrusion is formed. Such protrusions can be considered
problematic for at least certain applications. Such protrusions
also can be formed with FLA. With scanning mixed phase
solidification (MPS), as discussed below, those protrusions
generally are not formed. Instead, the resultant film has one or
more droplets in on top of the resultant film. These droplets can
be many times the film thickness (e.g., four or more), while
protrusions are typically less (e.g., four or less). The droplets
form because the excess liquid formed by the scanning is not
trapped in between two growth fronts, but rather is transported
along with the scanning beam through the liquid channels that exist
in between the growing crystals. Although pulsed MPS films are not
entirely smooth, a pulsed MPS does not have the droplet formation
of scanned MPS films.
[0056] Flash laser annealing uses a flash lamp to produce white
light over a wide wavelength range, e.g., 400-800 nm. The flash
lamp is a gas-filled discharging lamp that produces intense,
incoherent full-spectrum white light for very short durations. A
flash lamp annealing apparatus uses white light energy for surface
irradiation, in which the light is focused using, for example, an
elliptical reflector to direct the light energy onto a substrate,
such as is shown in FIG. 1. FIG. 1 is a simplified side view
diagram representing a flash lamp reactor 100 with a reflecting
device 110, in accordance with an embodiment of the present
invention. The flash lamp reactor may include an array of flash
lamps 120 located above a support 130, with a target area 150
situated between the two. The reflecting device 110 may be
positioned above the flash lamps to reflect varying amount of
radiation 160 from the flash lamps back towards different portions
of a facing side of the target area. The target area may be adapted
to receive a substrate (wafer).
[0057] The lamp power is supplied by a series of capacitors and
inductors (not shown) that allow the formation of well defined
flash pulses on a microsecond to millisecond scale. In a typical
flash lamp, light energy densities in the range of up to 3-5
J/cm.sup.2 (for a 50 .mu.s discharge) or 50-60 J/cm.sup.2 for a
1-20 ms discharge can be obtained. In exemplary embodiments, the
light energy density can be about 2-150 J/cm.sup.2. Flash lamp
annealing allows fast heating of solid surfaces with a single light
flash between some tens microseconds and some tens milliseconds,
e.g., 10 .mu.s-100 ms. Variables of the flash lamp that affect the
quality of thin film crystallization include the energy intensity
of the incident light, as well as the pulse duration and shape of
the light (which results in a certain dwell time, i.e., a duration
of melting).
[0058] Because flash lamp irradiation is a flood irradiation
process, the flash lamps can irradiate large areas of the substrate
surface in a single pulse. It is possible that the entire film on a
substrate, for example, a glass panel, can processed
simultaneously. Thus, multi-pulse operations in a scanned fashion
to cover a large substrate area, for example, as used in
laser-based recrystallization, are not required. However, the flash
lamp irradiation is not limited to full substrate irradiation, and
the flash lamp may also be shaped in a limited area, e.g., a line
beam to irradiate a selected region of the film. In one or more
embodiments, the substrate and flash lamp apparatus optionally can
be arranged so that the surface of the film is scanned and
sequentially exposed to light energy from the flash lamp apparatus.
Exposures may be overlapping to ensure complete crystallization of
the film. Exposures may further be overlapping by a large degree to
create multiple radiations per unit area while scanning.
[0059] Under certain irradiation conditions, liquid phases and
solid phases can coexist in the silicon film, and the
solidification process based on that melting regime is referred to
as "mixed phase solidification" or "MPS." In one or more
embodiments, irradiation using a flash lamp, diode laser in
divergent mode or other pulsed flood or divergent light source is
carried out under conditions to provide mixed solid and liquid
phases. These regions are solid or liquid throughout the thickness
of the film, although the overall irradiated surface includes
regions of solid and regions of liquid. The liquid phase may occupy
larger volume fractions than the solid phase. The solid phase
serves as seeding sites for formation of crystalline domains during
solidification and commonly growth of large <100> textured
grains is observed. In the MPS process, a near equilibrium is
established between the dynamically coexisting solid and liquid
phases. The balance between solid and liquid phases is used to
control the different characteristics of the crystalline grains
created after solidification. These characteristics include grain
size and grain orientation, specifically in the {100} surface
direction, and defect density.
[0060] In MPS, the film is partially molten in a way that is found
to favor {100} surface oriented grain growth at the expense of
other orientations which may disappear during the melting or, when
not eliminated during the mixed phase melting, which may undergo
less growth than the <100> grains during cooling and
solidification. Such orientation-dependent anisotropies in melting
and growth occur under close-to equilibrium conditions. Mixed phase
melting is established as a result of the difference in
reflectivity, R, between solid and liquid Si for wavelengths
roughly in the visible spectrum. Liquid Si has a higher
reflectivity than solid Si and tends to reflect incident light.
Provided the non-reflected light is sufficiently absorbed, the
difference in reflection results in solid regions being heated more
than liquid regions. This negative .DELTA.Q (Q is the heat
generated in the film, .DELTA.Q=Q(liquid)-Q(solid)) results in a
material in which liquids and solids are in a dynamic balance
wherein liquids are undercooled and solids are overheated.
[0061] In one or more embodiments, flash lamp annealing conditions
are controlled to provide a liquid content in the mixed phase
material that is greater than about 50 vol % liquid. The liquid
phase can approach 100 vol %, but complete melting of the entire
film should be avoided. In one or more embodiments, the liquid
phase is about 50 vol % to less than about 100 vol %, or about 80
vol % to about less than 100 vol %, of the mixed liquid/solid phase
during flash lamp irradiation.
[0062] <100> textured films are obtained through MPS provided
that {100} surface-oriented seeds are present prior to establishing
the mixed phase melting of the film. As used herein, "{100} surface
oriented grains or {100} seeds" means grains/seeds having
substantial {100} surface orientation, for example, within 5, 10,
15, or 20 degrees of the {100} pole. Thus, in one or more
embodiments, the film is pretreated to provide {100} surface
oriented grains or {100} seeds. Seeds may be created either during
deposition, if the precursor film is poly-crystalline; or, if the
precursor is amorphous, during post-deposition treatments (e.g.,
pulsed laser crystallization or solid phase crystallization) or in
the early stages of the crystallization process to induce MPS
(i.e., preceding the establishment of the mixed phase), for
example, via solid phase crystallization or via melt-mediated
explosive crystallization. The {100} seed content of the precursor
film affects the degree of melting as well as the dwell time that
is required to achieve strongly <100> textured films. For
randomly textured films, a large degree of melting and/or a longer
dwell time is required to achieve strong texture. For {100} surface
textured precursor films (e.g., available via certain CVD
processes), a lower extent of melting may be sufficient. See, U.S.
Ser. No. 10/994205, entitled "Systems and Methods for Creating
Crystallographic-Orientation Controlled Poly-Silicon Films," which
is hereby incorporated in its entirety by reference.
[0063] In order to achieve improvements in grain size and grain
texture, at least some melting of the film should occur. If the
energy density of the flash lamp irradiation is too low, no melting
will occur (at a certain dwell time) and the resultant film will
have small grain size and show little to no improvement in texture.
If less than 50 vol % liquid phase is achieved, then the mixed
phase is rich in solid phase seeding sites, but there is
insufficient melting to remove all non-{100} surface oriented
grains or to provide a significant increase in crystal growth. As
the volume percent liquid phase increases, a larger number of
grains will fully melt so that the grain size of the
re-crystallized grains will increase accordingly. However, if
melting in the irradiated region is complete, e.g., 100%, large
poly-Si grains will form as the grains grow laterally from the
unmelted solids located at or near the edge of the irradiated
regions. In addition, highly defective grains may form when the
liquid is allowed to become significantly supercooled (i.e., in the
absence of laterally growing grains) so that it solidifies via
nucleation of solids. While large polycrystalline grains may be
formed from the complete melt, the laterally grown regions are
commonly highly defective and exhibit poor-to-no preferred grain
orientation. Although not found in all instances, it is frequently
the case that re-crystallized films formed from a mixed
liquid/solid phase contain polycrystalline grains that are smaller
in size, but of lower defect density and greater texture, than
those formed from a complete melt recrystallization. In one or more
embodiments, the resultant film includes greater than about 90% of
the surface area of the film having an {100} surface orientation of
within about 15.degree. of the {100} pole. In other embodiments the
surface orientation is within about 10.degree., or about 5.degree.
of the {100} pole.
[0064] Multiple factors are considered when optimizing the
resulting seed layer. The dynamic balance of liquid and solid
during flash lamp irradiation can be maintained by control of the
lamp and beam properties and/or the irradiation conditions. The
light intensity (energy density), temporal profile of the light
exposure (pulse shape and dwell time) and light wavelength range
can be controlled. During flash lamp irradiation, processing
conditions such as the arrangement of the lamp (focus, etc.), the
equipment and irradiation implementation conditions, the scan
conditions, scan number, exposure number, substrate heating, film
preheating, co-irradiation and variable intensity exposure can be
controlled to obtain the desired melting and solidifying
conditions.
[0065] FIG. 2A is a cross-sectional illustration of the liquid 210
and solid 220 phases that can be generated in a film 200 of
homogeneous crystallinity or under steady state irradiation
conditions. Homogeneous crystallinity means that the crystals
arising from the liquid and solid regions have uniform orientation
(for example (100)) in the film 200 and contain few or no defects.
The liquid 210 and solid 220 regions are fairly regularly spaced
and the solid regions 220 are fairly uniform in size (as are the
liquid regions 210). As shown in FIG. 2B upon crystallization of
the liquid regions, the film 200 contains a higher proportion of
grains 250 having {100} surface orientation. The dimension of the
liquid phase can approach the critical solid-liquid coexistence
length (.lamda..sub.ls), which is the extent to which two phases
can exist before the mixed phase becomes unstable.
[0066] However, the critical solid-liquid coexistence length
(.lamda..sub.ls) is not a fixed length. Rather, it depends on
details of the irradiation and the sample configuration (i.e., film
thickness, thermal conductivity of film and substrate, which
influences heat removal) and the fraction of liquid in the film. A
graphical representation of .lamda..sub.ls 260 is shown in FIG. 2C.
The x-axis of FIG. 2C is fraction of liquid, i.e., how much liquid
is in the film. The y-axis is the solid-liquid coexistence length
(.lamda..sub.ls). The area above the curve 260 is the unstable
region 270. That is, the mixed solid liquid phase cannot exist at
those coexistence length and liquid fraction values. The area below
the curve 260 is the stable liquid solid coexistence region 280.
Values of the coexistence length and liquid fraction in the stable
liquid solid coexistence region 280 create a stable mixed
solid/liquid phase. Therefore, values of coexistence length and
liquid fraction can approach and equal the critical solid-liquid
coexistence length (.lamda..sub.ls) , but should not exceed it,
without the mixed solid/liquid phase becoming unstable. Preferably,
the mixed solid/liquid phase should be at or near the critical
solid-liquid coexistence length (.lamda..sub.ls).
[0067] Further, the value of the solid-liquid coexistence length
can vary based on the grain size of the thin film. For example, as
shown in FIG. 2A, films with large grains generally have a large
solid-liquid coexistence length. However, as shown in FIG. 3A,
films with small grains generally have small a solid-liquid
coexistence length.
[0068] In certain embodiments, the microstructure of a precursor
film allows the liquid/solid periodicity to reach a value
commensurate with this critical dimension. Going beyond that
critical dimension is not possible, but it is possible to select a
process that approaches or reaches .lamda..sub.ls. For mixed phase
systems with more than .about.50% liquid, a further increase in the
liquid fraction of the mixed phase system leads to longer
.lamda..sub.ls, as is discussed in greater detail below. When the
mixed phase becomes unstable (i.e., an unsustainable degree of
superheating in the solids and/or of supercooling in the liquids),
that situation typically will be rectified through melting or
growth to create liquid or solid regions within those unsustainably
superheated or supercooled regions, respectively, and regain near
equilibrium conditions. The growth of solids in this case does not
occur through nucleation as the degree of supercooling is
insufficient. Such an arrangement can also arise in a material that
is in a steady state irradiation, that is, in a material in which
liquids and solids are in a dynamic balance wherein liquids are
undercooled and solids are overheated.
[0069] FIG. 3A is a cross-sectional illustration of a heterogeneous
film 300 containing multiple grain boundaries 330 and grains 310,
320 of different orientations. The grains can also have different
levels of defectiveness. The melting of such a heterogeneous film
is influenced by preferred melting of grain boundaries, as well as
differences in melting behavior of the grains depending on their
crystallographic orientation and their defectiveness. The film will
form liquid 340 and solid 350 regions that are of varied spacing
from one another and of varying size, as is illustrated in FIG. 3B.
In addition, once a mixed phase is established, the complete
melting condition, or temperature, of a particular grain is
affected by the total fraction of solid within the heat diffusion
length of that grain, as well as to a curvature effect leading to a
higher melting temperature (Gibbs-Thomson effect). The different
grains in the heterogeneous film will thus have different local
melting temperatures (T.sub.m) that are a function of defectivity
density and orientation. Under uniform irradiation the film will
have a range of T.sub.m (T.sub.mas-T.sub.mm)and there will be a
slight but significant variation in the temperatures of the liquid
and solid regions, as is illustrated in FIG. 3B. It is found that
{100} surface oriented grains are most resistant to melting, but
other orientations, especially in the absence of {100} grains
nearby, may survive as well. When initially heating and melting a
heterogeneous film, the periodicity and size uniformity of the
liquid and solid regions may be compromised and the dimensions will
be smaller and will be related to nature of the precursor film.
Thus, the ability to readily form large domains of liquid depends
in part on the quality of the film. The solid-liquid periodicity
might, at least initially, be less than that for a homogeneous
film. Heterogeneous films may require longer dwell times and/or
multiple exposures to reach a mixed phase having dimensions
correlated to k.sub.is.
[0070] FIG. 4A illustrates the effect of a heterogeneous film 400
with low levels of grains 410 of the stable {100} surface
orientation and thereby high levels of grains of a different
orientation, e.g. surface oriented {hkl} grains 420, on the
formation of mixed phase regions. FIG. 4A is a cross-sectional
illustration of a heterogeneous film containing multiple grain
boundaries 430 and grains 410, 420 of different orientations. In
this case, there is a spacing between (100) oriented grains that is
greater than the critical solid-liquid coexistence length
(.lamda..sub.ls). Upon irradiation, the film will form liquid 440
and solid 450, 460 regions that are of varied spacing from one
another and of varying size, as is illustrated in FIG. 4B. In
addition, solid regions 450 and 460 can have different
crystallographic orientations. The critical solid-liquid
coexistence length is insufficient to form liquid regions bridging
(100) seeds and that that is why the {hkl} grain can survive, as
shown in FIG. 4C.
[0071] Seed crystals 420 having an undesired orientation may be
very difficult to remove when .lamda..sub.ls is short. Thus, when
using a heterogeneous film, even when a solid liquid periodicity
commensurate to critical solid-liquid coexistence length can be
achieved, this may not guarantee obtaining a highly textured film,
because the spacing between {100} oriented grains may be larger
than the critical solid-liquid coexistence length (or, stated
differently, the critical solid-liquid coexistence length is too
short).
[0072] In one or more embodiments, the film is subjected to
multiple FLA exposures. In some embodiments, the film surface may
be exposed twice or multiple times up to about one hundred or more
or a few tens times, and more typically is exposed about 2-10
times, or 2-4 times. As crystallographic texture is achieved over
multiple exposures, the annealing conditions can be selected to
produce a mixed phase composition that has a lower liquid content.
Thus, the flash lamp can be operated with lower intensity and/or
with shorter dwell times. Such conditions could be compatible with
thermally sensitive glass substrates. Multiple exposures can have
the advantage of resulting in larger-grained and more strongly
textured films. The improvement in average grain size with
increasing number of scans is illustrated pictorially in FIGS. 4C
and 5. Similarly, the anticipated increase in the level of (100)
texture (depicted at % {100}) is shown in FIG. 6. Thus,
multi-exposure processes tend to produce higher quality films.
[0073] In a first exposure, the solid liquid periodicity may not
yet reach a value dictated by .lamda..sub.ls. This could be the
result of the heterogeneity of the precursor film in which
defective grains or regions, including grain boundaries, or even
grains with certain orientations, may melt preferentially over
low-defect-density grains or regions and/or {100} surface oriented
grains. See, FIGS. 4A-4C. Thus, while some improvement in the grain
orientation and defectivity is observed in a single irradiation
process, inherent heterogeneity in the starting film does not give
rise to large periodic liquid and solid regions. Subsequent
irradiation of the marginally improved sample will provide a film
of increased {100} surface orientation and reduced defectivity. The
solid/liquid periodicity also may not yet reach a value dictated by
.lamda..sub.ls if the initial microstructure of the precursor film
is on a scale much smaller than .lamda..sub.ls. In such
circumstances, a mixed phase is created with a periodicity on the
same scale as the microstructure, as it generally takes time for
the mixed phase to evolve. This will be particularly the case in
situations where a short dwell time is preferred (e.g., for
substrate compatibility) and in those cases a multiple pulse
process may be used to sequentially increase the grain size and the
texture of the films. The resultant films have a high level of
(100) grains and the grain size is generally larger than that
achieved with single exposure.
[0074] Depending on the application, a single exposure technique
may be sufficient. Because single exposure techniques require
approaching complete melt conditions, the multi-exposure techniques
afford more freedom and the factors can be adjusted within a wider
window of operation. In fact, the difference in degree of melting
desired in a single-pulse or a multiple pulse process may not be
all that large. While a lower degree of melting may be possible
(e.g., 90 to 95% instead of 99% or approaching 100%) in multiple
exposure methods, the real gain from multiple exposures is the
gradual elimination of the non-(100) grains while also increasing
the liquid/solid periodicity. Also, subsequent radiations need not
be at the same energy density, for example, the energy densities
may be different to accommodate changes in the optical properties
of the film (e.g., due to phase change or change in defect
density), or to optimize the sequential increase in grain size and
texture.
[0075] For example, experimental observation has shown that the
second and subsequent pulses in a multiple pulse process, starting
with an amorphous or highly defective precursor, can actually have
an energy density as much as twice that of the first irradiation
pulses. This is related to the use of longer wavelength light at
which transparency shifts between amorphous and crystalline are
much larger. Therefore, the second and/or subsequent pulses may
need significantly higher energy, e.g., twice, or at least more
than 20% more energy than the first pulses. This difference is much
larger than previously observed during work on scanning-mode MPS
where shifts on the order of a few percent, but no more than 20%
were used.
[0076] In one or more embodiments, a thin seed layer thin film is
exposed to multiple exposures in a pulsed flood or divergent
irradiation process to not only reach grain sizes commensurate with
.lamda..sub.ls, but also to clean up the material and remove
non-(100) grains. As is described herein, a single exposure may
lead to small non-(100) grains located at or near grain boundaries.
See, FIGS. 4A-4C. While for some applications/situations this may
be acceptable, it is not the most optimal. These grains are very
hard to remove without resorting to multiple exposures. This may be
due the use of a heterogeneous precursor where a solid-liquid ratio
may be established based on the small grain size and large spacing
between (100) seeds and a non-(100) seed, which may survive simply
because the distance between the (100) seeds exceeds .lamda..sub.ls
even permitting time for establishing a periodicity commensurate to
.lamda..sub.ls, even when there is time for establishing a
periodicity commensurate with .lamda..sub.ls (long dwell time).
[0077] In another embodiment, a second FLA pulse can be spaced
close enough to the first FLA pulse in the time domain that the
film is still at elevated temperature from the first radiation,
although it could be substantially solidified, when it is hit with
the second radiation. Thus, the reduced energy requirements for the
second pulse due to the residual temperature may lead to larger
.lamda..sub.ls. In this embodiment, there may be a need for two
(arrays of) flash lamps to allow pulses closely following each
other.
[0078] During FLA, the discharge lamps can provide light energy as
a discharge current pulse, wherein the pulse full width at half
maximum (FWHM) can range from less than tens of microseconds to
more than tens of milliseconds. For multiple irradiations, the
frequency of the pulses can also be controlled and typically can
vary in the range of hundreds of hertz. Dwell time is the time from
the onset of melting to full solidification. In continuous waveform
(CW) techniques, the dwell time is largely influenced by the
spatial profile of the laser beam and may further be influenced by
heat diffusion away from the scanned laser. In FLA techniques or
other flood or divergent irradiation techniques, the dwell time is
mostly influenced by the temporal profile of the flash lamp. Also,
dwell time may be influenced by various means of preheating.
[0079] As the dwell time is increased, the texturing process may be
more pronounced, but the substrate is also exposed to light energy
for a longer duration. The thermal diffusion coefficient transports
the heat through the film thickness. Longer dwell times, while
improving the quality of the grain size and texture of the seed
layer, may cause heat to undesirably transport into the substrate,
which is problematic for heat sensitive substrates.
[0080] A further feature of the flash lamp is the light energy
density of the incident light, which depends on the input energy of
the flash lamp, can be controlled by varying the voltage and
capacitance of the flash lamp. Light energy density will vary with
the particular flash lamp apparatus that is used (e.g., pulse
duration and pre-heating), but can typically vary in the range of
less than about 2 to 150 J/cm.sup.2 or more. The energy intensity
is desirably above a threshold level I.sub.1 in order for melting
and mixed phase recrystallization to occur. Below the energy
threshold I.sub.1, the film does not form any liquid phase and
improvements to grain size and texture are poor, even at long dwell
times. The light intensity is also desirably below an upper
intensity I.sub.2, at which the film melts completely. At high
energy intensities, I.sub.2, the exposed area will melt completely
and the benefits of mixed phase recrystallization are not
observed.
[0081] Another factor in controlling the beam quality is related to
the wavelength range of the incident white light. As noted above,
mixed phase melting is established as a result of the difference in
reflection between solid and liquid for wavelengths roughly in the
visible spectrum. The liquid phase exhibits higher reflectivity.
Provided the non-reflected light is sufficiently absorbed, the
difference in reflection results in solid regions being heated more
than liquid regions, which is a necessary condition for the mixed
phase melting and solidification to occur.
[0082] Different light sources will have their own unique
wavelength range which will be absorbed by the film. Commonly used
light sources in Si film crystallization radiate at short
wavelengths, for example, UV light from excimer lasers (e.g., 308
nm for XeCI) or medium wavelengths, for example, frequency doubled
diode-pumped solid state lasers (e.g., Nd:YVO4 at 532 nm). These
wavelengths absorb entirely (for UV) or sufficiently well (for
green 532 nm) in Si. Longer wavelengths may not absorb well enough
and are not efficient for crystallizing thin Si films (for optical
data on absorption in Si, see for example the 88.sup.th edition
(2007-2008) of the CRC Handbook of Chemistry and Physics, section
12, p 12-1 38, which is incorporated herewith by reference). The
light from flash lamps also contains much longer wavelengths (a Xe
gas discharge lamp produces white light in the range of 400-800 nm)
and the light of diode lasers may be exclusively consist of long
wavelengths (e.g., .about.808 nm). An appropriate mixed phase can
for instance be achieved using 532 nm light. Even so, at this
wavelength, the Si film may already be partially transparent
(depending on film thickness and interference effects) and some
thicknesses are better suited than others for inducing MPS.
[0083] As a result of these transmission losses (which are expected
to be higher for the semiconducting solid Si than for the metallic
liquid Si), for longer wavelengths it will become progressively
more difficult to get a sufficiently negative .DELTA.Q to induce
MPS, even though the change in reflectivity .DELTA.R is still
positive (.DELTA.R=R(liquid)-R(solid)). In one or more embodiments,
a metallic layer is used underneath the Si layer as a heat
absorption layer. The heat of the incident light that is not
absorbed by the Si layer is absorbed instead by the underlying
metal layer and thermally diffuses back into the Si layer. The
metal layer can be any metal having the appropriate thermal
absorption. By way of example, the metal layer can include a
molybdenum film deposited prior to Si deposition (with a possible
barrier in between) or it could be a metallic substrate (e.g., a
flexible stainless steal substrate for making flexible large area
electronics such as solar cells or AM-OLEDs). In one or more
embodiment, the metal does not negatively interact with the Si
layer, for example, by poisoning the layer. In other embodiments, a
barrier layer is disposed between the metal layer and the Si
substrate. In one or more embodiments, a metal film is provided
only in selected areas (e.g., using lithographic processes) so that
MPS can be induced in those selected areas only while in other
areas less light gets absorbed resulting is less heating.
[0084] In one or more embodiments, other efficient pulsed light
sources may be used for the MPS process. One such example is a
diode laser, which is capable of pulsed lasing at for example
.about.800 nm and which has been previously been used to induce
melting in a process referred to as diode laser thermal annealing.
See, e.g., Arai, et al., "41.2: Micro Silicon Technology for Active
Matrix OLED Display," SID 07 Digest, pp. 1370-1373 (2007) and
Morosawa, et al., "Stacked Source and Drain Structure for Micro
Silicon TFT for Large Size OLED Display", IDW, pp. 71-74 (2007),
which are incorporated herein by reference in their entirety. High
power diode lasers can be power efficient and can have high
divergence, making them more lamp-like than most other lasers.
Their divergence makes them more suitable than other lasers to be
placed in arrays to establish uniform 2-D heating of a film. Diode
lasers can also be pulsed and the short pulse durations that can be
achieved may be beneficial for reaching compatibility with low-cost
substrates, such as glass. A metal layer underlying the silicon
film may be required in order to sufficiently absorb the light of a
diode laser due to the longer wavelength of light and to
successfully establish mixed phase melting and solidification. In
one or more embodiments, a metal layer may be used even with
wave-lengths of light that absorb well, in order to achieve desired
heating effects. The metal layer may further be useful to smear out
non-uniformities in the radiation from the diode laser as can for
example result from the coherence of the light. The metal layer is
very conductive and may redistribute heat from hot spots to cooler
regions nearby on a time scale shorter than, or comparable to, the
time required to establish a mixed phase. The metal layer may also
be patterned to induce MPS only in desired areas.
[0085] In the mixed phase melting and solidification regime, a
critical solid-liquid coexistence length (.lamda..sub.ls) can be
recognized beyond which the mixed phase becomes unstable as a
result of the degree of superheating and undercooling of the solids
and liquids respectively reaching unsustainably high values. As a
result, the mixed phase will evolve into an approximately periodic
structure consisting of superheated solid regions alternating with
undercooled liquid regions. See, FIG. 4. The periodicity is linked
with .lamda..sub.ls, which in turn will be determined based on the
details of radiation, pre-heating, and heat flow in the film, as
well as the degree of melting established; a simple analysis has
been provided previously in Jackson, et al. "Instability in
Radiatively Melted Silicon Films," Journal of Crystal Growth 71,
1985, pp. 385-390, the contents of which are incorporated in their
entirety by reference. As growth proceeds from the solid regions
into the liquid regions, it follows that the grain size will
generally tend to saturate at values around .lamda..sub.ls. As
there is a dependence of .lamda..sub.ls on the liquid fraction,
larger grains can be obtained by radiation at a condition close to
complete melting, e.g., under condition of large liquid
content.
[0086] In situations where the crystallinity of the seed layer is
not homogeneous, e.g., there is a variation in the orientation and
defectivity of the grains, the mixed phase periodicity of liquid
and solid may not be uniform. In addition, the liquid regions may
be smaller than .lamda..sub.ls due to the presence of
preferentially melting grain boundaries that interrupt the optimal
formation of the liquid phase. In one or more embodiments, the
flash lamp irradiation process is selected to increase
.lamda..sub.ls, increase grain size and reduce defectivity.
[0087] Various techniques can be used to increase the coexistence
length so as to approach .lamda..sub.ls. One technique involves
lowering the intensity of the incident light. The intensity of
radiation can be reduced by reducing the rate of loss of heat
towards the substrate or the surroundings. In one embodiment, by
using flood pulsed annealing of a large section of the film, there
are no significant lateral temperature gradients and less intense
radiation suffices to establish MPS. In further embodiments, lower
intensity radiation may be established by sample pre-heating, e.g.,
via co-irradiation from front or back side or via hot-plate
heating, or by increasing the pulse duration. Further, the use of
pulsed MPS as opposed to line-scanned MPS reduces the lateral heat
loss and thereby increases .lamda..sub.ls.
[0088] The temporal profile of the beam also may be controlled to
improve the degree of (100) texture. Even when a light irradiation
technique achieves co-existence of solid and liquid phase, it may
not result in a desired quality of crystalline growth. Growth may
take place at a condition progressively further removed from
equilibrium and the growth may be more defective due to defect
formation and orientation roll off. Thus, a factor in increasing
the quality of {100} surface-oriented grains in the film is
controlling the speed of ramping down the pulses. In "beam off"
crystal growth, the energy density changes (decreases) abruptly and
cooling and crystallization takes place in the dark, e.g., with the
light beam off Beam-off crystal growth can have a strongly facetted
nature, but may also quickly result in loss of orientation through
twinning, defective growth, and/or orientation roll off. So, even
though the mixed phase formed during irradiation may predominate a
material having a {100} surface orientation, once it cools down the
orientation may not be preserved.
[0089] In one or more embodiments, the {100} surface orientation is
obtained using a "beam on" temporal energy profile. In "beam-on"
crystal growth, radiation of the film (albeit at decreasing
intensity) is continued after mixed phase formation. The
near-equilibrium condition is maintained longer during the
solidification and the quality thereof is higher as well as having
stronger preferential growth of {100} surface oriented seeds over
other orientations. In beam-on solidification, the growth of solid
seeds may itself become subject to the mechanisms that result in
the formation of the mixed phase and, as a result, the growth front
may not be facetted but may become cellular or even dendritic in
nature to maintain a solid-liquid periodicity commensurate with
.lamda..sub.ls. The periodicity of the cellular growth front will
further be affected by the reduction in .lamda..sub.ls as the
liquid content decreases. Such modes of growth need not result in
defective material but ultimately are characteristic of material
having typically at least low-angle grain boundaries.
Considerations of beam-on and beam-off solidification scenarios
lead to an engineered temporal beam profile that may establish a
trade-off between the extreme scenarios experienced in either, as
well as in the maximum extent of melting that is induced.
[0090] Exemplary suitable beam-on conditions may be determined
empirically or by using crystallization modeling. In one
embodiment, a Si thin film is irradiated at a peak power to produce
a large volume fraction of liquid, i.e., near complete melt. After
that, for beam-on radiation, the light power is gradually reduced
until complete solidification has occurred. The complete
solidification time depends on growth velocity. Growth velocities
in silicon can be up to more than 10 m/s as for example encountered
in pulsed-laser induced lateral growth using excimer lasers with
10s or 100s of nanosecond pulse duration. For the present method,
longer pulse durations are envisioned and velocities may be more on
the order of 1 cm/s to 1 m/s. Then, assuming growth distances of 1
or up to 5 or 10 .mu.m (depending on solid-liquid periodicity),
this would mean a gradual ramp down of 1 .mu.s to 1 ms. In general,
before substantial solidification has occurred, the power is
lowered to between 40% and 90% or between 60% and 80% of the peak
power of the flash lamps. Hawkins and Biegeleson (Appl. Phys.
Lett., 42(4), February, 1982 pp. 358-360) which is incorporated in
its entirety by reference, show the relationship between silicon
temperature and laser power and indicate a plateau at which
liquid/solid mixed phases coexist.
[0091] Without being bound by any particular theory or mode of
operation, one reason why the growth in beam-on crystallization is
believed to have a low defect density is related to the temperature
gradients in the film. In pulsed laser crystallization, e.g.,
directional sequential lateral solidification, there are typically
very strong temperature gradients in the region behind the growth
interface. These result in temperature-gradient induced stresses
which are believed to be the source of defect formation through
plastic deformation; especially of low angle grain boundaries that
rapidly devolve into higher angle grain boundaries (Crowder et al,
Mat. Res. Soc. Symp. Proc. Vol. 685E, 2001 Materials Research
Society, which is incorporated in its entirety by reference).
Beam-off crystallization resembles this in that the solid cools
rapidly resulting in strong temperature gradients in the region
behind the lateral growth front. In beam-on crystallization, on the
other hand, the solid is constantly heated so there is a smaller
lateral temperature gradient which furthermore is inverted at the
interface since the solid absorbs more than the liquid. Without
being bound to any particular theory or mode of operation, this may
be the reason why no defects are formed at or near the growth
front.
[0092] Preheating can be used to raise the base temperature of the
film so that less energy or shorter pulse times are required to
obtain the desired level of liquid/solid mix. Pre-heating mechanism
include use of a heated substrate, such as a hot plate and
co-irradiation, in which one radiation is used for heating and a
second irradiation is used for preheating. By way of example, an
exposure having a long pulse duration of low intensity is used for
heating and then an exposure having a short pulse duration of high
intensity is used for MPS processing. The co-irradiation can be
from the same side, or opposite sides. In other embodiments, the
film is preheated by irradiation from the side opposition the
film.
[0093] Another controlling factor is the number of times the film
is exposed to the light. Some applications use a single exposure
(per unit area), while others use multiple beam irradiations to
crystallize the film. For solar cells, both single and multiple
irradiation methods may be used.
[0094] In one or more embodiments, the silicon film is subjected to
a single FLA exposure. In order to achieve strong crystallographic
texture in a single exposure, annealing conditions are selected to
produce a mixed phase composition that is close to complete
melting, e.g., greater than 80% vol. or greater than 90% vol.
liquid. Exemplary process conditions include preheating the
substrate to a high substrate temperature (in the case of a silicon
film, for example, to about 400.degree. C. to 1200.degree. C. or
600.degree. C. to 900.degree. C.) and using a beam temporal
profile, including slow heating and cooling, which brings the
crystal close to full melting and creates large crystals that
predominately have {100} surface orientations. To achieve higher
levels of liquid and larger coexistence length, e.g., approaching
k.sub.is, the flash lamp is operated at low power, i.e., to provide
a lower intensity light energy to the film surface, so that the
system can be slowly heated and cooled, e.g., longer pulse dwell
times at lower pulse intensity. Recognizing that different
materials and conditions will provide different specific outcomes,
it is generally observed that the resultant poly-Si films have high
levels of (100) grain texture, but that other grain orientations
are also present. Other orientations may exist as small grains from
seeds that were located far away from {100} surface oriented seeds
at the peak of mixed phase melting, by virtue of which they may
have survived the mixed phase melting in the first place, but have
undergone little or no growth during solidification due to the
anisotropies in growth at near-equilibrium conditions. These small
and possibly more defective grains are typically observed at or
near grain boundaries (i.e., far from the seeds that led to large
{100} grains) and are considered less harmful for solar cell
applications (where the grain boundary region is already a region
with shorter carrier lifetimes).
[0095] Because of the longer dwell time, there may be significant
substrate heating and such methods are suited for thermally stable
substrates, such as certain metal and ceramic substrates. While
such substrates may not be acceptable for all applications, such as
for example in display TFTs where substrate transparency is
desired, no such limitation is required for solar cell
applications. In one or more embodiments, steps are taken to avoid
overheating the substrate, which can arise by thermal diffusion
over the longer pulse dwell time, for example, by limiting the area
of heating (e.g., using localized heating by patterned metal
absorption layers or by patterned reflective metal layers on top)
or by using thick buffer layers that may further have very low
thermal conduction (e.g., porous layers).
[0096] In the techniques using flash lamps with flood exposure,
repeated exposure only requires flashing the lamp more than once.
With every new flashing, a portion of the crystal grains are
destroyed and re-solidified from neighboring seeds. Thermodynamic
factors involved include interaction between defective and less
oriented grains and less defective and more oriented grains.
[0097] FIGS. 7A and 7B are in-situ photomicrographs of an Si thin
film that is being crystallized using partial melt processing and
CW complete melting, respectively. The film is being exposed to CW
at a very slow scan rate CW scan, which is less relevant to partial
melt processing; however, it is illustrative of what happens as the
fraction of liquid decreases. The image in FIG. 7B shows complete
melting. On the left side designated by arrow 700 there is clear
cellular directional growth. Close to the complete melt region, at
arrow 710 the solid liquid spacing is double that closer to the
solidified region. Something similar happens with films subjected
to partial melting as illustrated in FIG. 7A. As can be seen at
arrow 720, the grains grow away in lamellar shapes to meet the
periodicity commensurate with .lamda..sub.ls, which decreases with
decreasing liquid content.
[0098] Traditional aluminum-induced crystallization techniques
result in large grains having a high number of intra-grain defects.
Thus, the resulting crystalline light absorption layer behaves like
a material having a much smaller grain size. The resulting grains
might be smaller than those produced by traditional methods, but
the grains advantageously also have a lower density of defects and
thus are more suitable for solar cells. The seed layer includes a
silicon layer having a thickness of about 50 nm to 1 .mu.m (or even
thicker) or 150 nm to 500 nm having a low defect density and high
degree of (100) textured grains. By way of example, the seed layer
suitable for use in solar cells will have more than 90% or 95% or
even 98% of the surface of the sample having an orientation within
15.degree. of the {100} pole. The seed layer is prepared as
described above.
[0099] The subsequent step, epitaxial growth of a thicker silicon
layer, traditionally takes place at high temperatures, above
600.degree. C. Recent low temperature techniques use hot wired CVD
deposited layers and can be performed at around 600.degree. C.
These low temperature techniques are preferred to the high
temperature techniques because of compatibility with lower-cost
substrates. At the same time, the low temperature techniques, more
than the high temperature versions, require a (100) textured seed
material to result in proper epitaxial growth. Exemplary thickness
of the epitaxially-deposited layer is between 1.5 .mu.m to 20 .mu.m
or between 2 .mu.m and 6 .mu.m.
[0100] The seed layer approach is also advantageous in growing a
solar cell's p-n junction or dopant gradient. The absorber layer
can be grown with a different doping species and/or different
concentration thereof from the seed layer and furthermore can be
provided with a gradient in doping concentration by varying the
relative concentration thereof in the deposition gas mixture. In
this way, the p-n junction of the solar cell can be introduced. The
epitaxially grown layer may also have the same doping species
throughout as the seed layer and a p-n junction is later formed in
a subsequent deposition step to create an emitter layer that is
possibly in the amorphous phase. The absorber layer can have a
different level of dopant concentration or even a gradient thereof
to produce a back surface field for reducing minority carrier
recombination at a back contact. The seed layer can be highly doped
to simultaneously act as a back contact for the solar cell.
[0101] In one or more embodiments, the epitaxial growth phase can
be prepared using epitaxial explosive crystallization. Epitaxial
explosive growth takes advantage of the relative thermodynamic
stabilities of amorphous and crystalline silicon to initiate and
propagate an epitaxial crystalline phase through the thickness of
the silicon layer. Further details of the method are found in
co-pending application Ser. No. 61/012,229, entitled "Methods and
Systems for Backside Laser Induced Epitaxial Growth of Thick Film",
which is hereby incorporated by reference in its entirety. One
advantage of the proposed technology is that the seed material is
almost fully textured in a (100) orientation, which is advantageous
in the use of epitaxial explosive growth techniques.
[0102] Solar cells can use glass, as well as non-glass substrates.
While the MPS methods can be used on non-glass substrates, they
have to be optimized to meet the limitations of glass substrates.
On the other hand, these methods are appropriate for stainless
steel or ceramic substrates. FLA technology can be used on both
glass and non-glass, e.g., stainless steel or ceramic,
substrates.
[0103] The present application does not require using the SLS
techniques. Nevertheless a hybrid mechanism combining the mentioned
techniques with the SLS methods can be envisioned. MPS may result
in a uniform grain size material. This is desired for optimum solar
cells. SLS may further be used to create more uniform grain size
films, as well as to further increase the grain size. Even though
far-from-equilibrium lateral growth is known to typically result in
defective growth (through twinning, stacking faults, or even
complete breakdown of epitaxial growth into highly defective
material), for (100) surface textured material it is known that
substantially defect-free material can be achieved over at least a
significant lateral growth length.
[0104] Also, the techniques may further be used to create (100)
textured films for use in 3D-ICs, for example, using the hybrid SLS
process or previously disclosed processes (or any derivative) to
create location-controlled single-crystal islands as, for example,
described in Song, et al., "Single-crystal Si islands on SiO.sub.2
obtained via excimer-laser irradiation of a patterned Si film,"
Appl. Phys. Lett. 68 (22), May 1996, pp. 3165-3167, which is hereby
incorporated in its entirety by reference.
[0105] Additionally, FLA can cause unwanted lateral crystallization
in a thin film. This can occur when the lateral growth or explosive
crystallization extends beyond the region being irradiated.
Therefore, when irradiating a film with FLA, the film can have good
quality crystallization sections, corresponding to the region being
irradiated, and poor quality sections, corresponding to the
unwanted lateral growth. Also, these unwanted lateral growth
regions also have different optical properties from the properly
crystallized regions, which can complicate later irradiation
processes. Therefore, in some embodiments, for example, shown in
FIGS. 8A and 8B, the unwanted lateral crystallization can be
reduced by providing barriers for lateral heat flow at the edges of
the radiated region of a thin film 800 on a substrate 805. The
barriers or isolation of the film can be provided by etching the
thin film 800 or by also etching the underlying layers, for
example, a buffer layer 810 (as shown in FIG. 8A). The etching of
the thin film reduced irradiation heat transfer between a first
section 801, a second section 802 and a third section 803. However,
some heat may be transferred through the substrate. Therefore, as
shown in FIG. 8B, the substrate 805 can have one or more trenches
815. These trenches 815 can further reduce heat flow between the
first section 801, the second section 802 and the third sections
803, thereby further limiting unwanted lateral crystallization.
Such trenches 815 can be made using conventional etching techniques
or even laser scribing techniques.
[0106] This embodiment can prevent non-sharp/smeared crystallized
domains. In other embodiments, because of long heat diffusion
length, wide edges that are non-uniformly crystallized can form,
which may prevent close tiling. For example, once a region is
crystallized via explosive crystallization, the optimum energy to
induce mixed phase solidification has shifted and a next radiation
may thus not lead to MPS in those explosive crystallization
regions. This process allows for more sharply defined crystallized
regions.
[0107] Upon review of the description and embodiments of the
present invention, those skilled in the art will understand that
modifications and equivalent substitutions may be performed in
carrying out the invention without departing from the essence of
the invention. Thus, the invention is not meant to be limiting by
the embodiments described explicitly above, and is limited only by
the claims which follow.
* * * * *